Field of the Invention: This invention relates to laminated windows and a method for maintaining the clarity of laminated windows, particularly laminated windows used in motor vehicle glazing containing a polyurethane interlayer which has one major surface exposed to the atmosphere. More particularly, this invention relates to retarding bloom in such laminates when the polyurethane interlayer normally blooms upon exposure to the atmosphere.
Description of the Prior Art: Safety glass for motor vehicle glazing, such as windshields, has for some time been made in a trilayer configuration; namely, two outer layers of glass bonded together with an interlayer of polyvinyl butyral. Although the trilayer safety glass windshield is safer than the older monolithic glass windshield, the trilayer is, unfortunately, a principal source of both lacerative and concussive injury in automobile accidents. During the course of an accident, an impacting passenger's head may shatter the inner glass ply of the windshield to form jagged slivers of glass which can severely lacerate the face and forehead of the impacting passenger.
In attempts to improve the safety performance of the trilayer safety glass windshield, the prior art has disclosed various bilayer windshields composed of an outer ply of glass and an inner ply of flexible plastic. The exposed inner plastic ply serves as a spall shield, shielding the passengers from any jagged slivers of glass which may result during the course of an accident.
Typical of the prior art relating to bilayer windshields is the following: U.S. Pat. Nos. 1,128,094 to Benedictus; 1,342,267 to Mascart; 2,047,253 to Brooks; 2,143,482 to Herrmann et al.; 2,184,876 to Sherts; 2,234,829 to Neher et al.; 2,454,886 to Saprio; 2,489,026 to Gilbert; 2,526,728 to Bert et al.; 3,157,563 to Baum; 3,532,590 to Priddle; 3,575,790 to Fleck; 3,580,796 to Hick et al.; 3,626,023 to Brizgys; 3,781,184 to Domicone et al.; 3,805,985 to Hagiwara et al.; 3,806,387 to Peetz et al.; and 3,808,077 to Rieser et al.; and South African Pat. No. 73.3884.
Some of the most promising work in the development of safety glass involves the use of polyurethane interlayers. See, for example, U.S. Pat. Nos. 3,388,032 to Saunders; 3,509,015 to Wismer et al. and 3,620,905 to Ahramjian, and Belgian Pat. No. 785,125, all of which disclose the use of polyurethane interlayers in fabricating safety glass. The polyurethanes have optical clarity and have excellent impact resistance over a wide temperature range.
There is, however, a disadvantage associated with certain of the above-mentioned polyurethanes. A particular disadvantage is that certain polyurethane formulations develop "bloom", characterized by a hazy surface appearance, upon exposure to the ambient atmosphere. The bloom is a surface deposit which is believed to be derived from low molecular weight material leaching to the surface of the polyurethane sheeting. This bloom scatters light and reduces the transparency of the interlayer and of any laminates prepared therefrom with the surface of the polyurethane exposed.
Although the bloom can be removed by dry wiping or solvent wiping, the bloom often recurs after removal. With bilayer windshields, bloom is a problem because one surface of the polyurethane is exposed to the atmosphere.